1. Field of the Invention
This invention relates to the preparation of tin-titanium complexes. More specifically, this invention relates to a process of preparing esters and polyesters in an improved manner by using these novel complexes as esterification/transesterification catalysts.
2. Prior Art
The preparation of esters and polyesters is well known in the art. These materials are prepared by esterifying mono- and/or polyhydric alcohols with mono- and/or polycarboxylic acids. In many instances it is desirable to prepare such esters or polyesters having a very low acid number, i.e., less than two. However, the attainment of an ester or polyester having a low acid number is extremely difficult to achieve. For example, during the last stage of the esterification of the reaction, the reaction proceeds extremely slowly and hence in order to reduce the acid value it is necessary to heat for a lengthy period of time at relatively high temperatures. This, in many instances, tends to bring about formation of a darkened ester or polyester which is unsuitable for further use. Various attempts have been made to reduce the time of the esterification reaction. For instance, British Pat. No. 792,011 describes the use of iron, cadmium, cobalt, lead, zinc, antimony and manganese in the form of the metal, its oxide or its salt with a bibasic acid. Other catalyst types are more reactive. For example, stannous compounds of the type Sn(OOCR).sub.2 wherein R is selected from the group consisting of saturated and unsaturated aliphatic hydrocarbon constituents having from about 7 to about 17 carbon atoms have been described in U.S. Pat. No. 3,162,616 and U.S. Pat. No. 3,716,523. Organo titanium or organo zirconium compounds have been disclosed by U.S. Pat. No. 3,056,818 as being suitable as esterification catalysts. Titanium compounds are often insoluble in the product and must be removed by filtration or other tedious procedures. The technical literature also notes that esterification is usually effected by refluxing the acid and alcohol with a small amount of acid catalyst such as sulfuric acid, hydrochloric acid, and sulfonic acids or boron trifluoride. Acid catalyst often causes color formation and must be removed to insure a stable low acid number product.
Esters may also be prepared by transesterification reactions. These transesterification reactions include the reactions between two esters to yield two new esters or the reaction between an ester and an alcohol to form a new ester and liberate an alcohol. Included also are the transesterification reactions where the components of the esters involved are polyhydroxy alcohols and polybasic acids. These reactions may, in some instances, be catalyzed by those substances which are employed for the esterification procedure. We have discovered that polyesters having an acid number less than two can be prepared in relatively short period of time with tin-titanium catalyst complexes employing either esterification or transesterification procedures.